Practice Problems and Solutions

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Practice Problems and Solutions

• http//linus.highpoint.edu/atitus/mandi/

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Origin of Charge

• Two types of charge
• Positive (protons, )
• Negative (electrons,-)
• Opposite charges attract
• Like charges repel

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Macroscopic Objects

• Objects are generally neutral.
• Equal numbers of and charges.
• Net charge is zero.

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Net Charge 0
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Positive Net Charge

• What if we remove some negative charges?

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Net Charge 0
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Positive Net Charge

• What if we remove some negative charges?
• Object has an overall positive charge.
• How could we get a net negative charge?

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Net Charge
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Negative Net Charge
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• Add negative charges.
• Object has an overall negative charge.
• But how do we add or remove charges?

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Net Charge -
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Magic Tape

• Friction is a good way.
• Take two neutral pieces of tape.
• Stick the sticky side of one piece to the
  non-sticky side of other.
• Rip apart quickly.

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Conservation of Charge

• Negative charges moved from one neutral piece of
  tape to the other.
• The total charge on both pieces together is still
  zero.
• The net amount of charge produced in any process
  is always zero.

Equal amounts of positive and negative charge
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Experiment
Estimate the amount of charge on a piece of
Scotch tape.

• Place one long, piece of tape on the table (with
  tab on the end). (tape L)
• Place a second piece on top of the first piece.
  (tape U)
• Gently lift tape L from the table, and place it
  across two books to make a bridge (make sure it
  is loose, like a hanging bridge).
• Lower the U tape until the L tape is in
  equilibrium. (FgravFelec)

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Results
How many atoms on a piece of tape lost an
electron?
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Types of Materials

• Conductors (metals)
• Some electrons are loosely bound to nuclei and
  can move freely in the material.
• Insulators
• All electrons are tightly bound to nuclei.

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Model of a neutral atom
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Affect of an electric field on a neutral atom
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Atom is polarized due to the electric field at
its location
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Representation of a polarized neutral atom
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Representation of a polarized insulator
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Neutral Conductor and Charged Insulator
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Interaction of a charged insulator and a neutral
conductor

• Free electrons are repelled from rod.
• Right side charged left side - charged.
• Net attractive force between rod and conductor.

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Interaction of a neutral insulator and a charged
insulator
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Induced Charge in an Insulator

• Negative charges repelled by rod.
• Electrons are not free but polarization occurs.
• Face of insulator becomes charged.
• Net attractive force between rod and insulator.

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Units of Charge

• The SI unit of charge is the Coulomb (C).
• Charge is quantized
• Charge comes in discrete amounts.
• Smallest known charge is that of an electron.
• Elementary charge e 1.610-19 C.
• Charge on electron -e -1.610-19 C.
• Charge on proton e 1.610-19 C.

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Calculating Net Charge (Example)
Given an object that is composed of 2.01011
protons and 4.01011 electrons, what is the
objects net charge?
Charge due to protons (protons)(charge per
proton) (2.01011
protons)(1.610-19 C/proton)
3.210-8 C
Charge due to electrons (electrons)(charge per
electron)
(4.01011 electrons)(-1.610-19 C/electron)
-6.410-8 C
Net charge Sum of all charges
(3.210-8 C) (-6.410-8 C)
-3.210-8 C
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Example
Rubbing a plastic rod against a piece of
wool transfers 6.0104 electrons from the wool
to the rod. What is the net charge of the rod?
(6.0104 electrons)(-1.610-19 C/electron) -9.61
0-15 C
What is the net charge of the piece of wool?
9.610-15 C